The following invention relates to a magnetizing device for producing permanent magnetic fields in magnet material and more particularly to a magnetizer in which the intensity and duration of a magnetizing pulse may be controlled by regulating the amount of stored charge used to create the pulse and the voltage level of the devices used to store the charge.
Permanent magnets of the type used in electric motors, linear actuators and the like are created by magnetizing magnetically hard permeable materials. This type of magnetization is usually accomplished by placing the material within a high intensity magnetic field which is created by passing a very high electric current pulse through a coil or coil-pole structure usually called a magnetic fixture. The current pulse is created by the sudden release of charge stored in charge storage devices such as banks of capacitors. The high current pulse resulting from the sudden release of the charge passes through the windings of the magnetic fixture and causes a brief but very strong magnetic field which, in turn, causes the magnetic domains within the magnetically permeable material to align in the required pattern. This alignment is achieved within the material in such a brief period of time (less than a few millionths of a second) that it may be considered instantaneous. Once the magnetic field of the fixture collapses, the material remains permanently magnetized.
Magnetizing fixtures and coils have considerable inductance which tends to slow the rate of current build-up after the pulse has begun. In addition, eddy currents may be present in the pole materials of the fixture, in the windings, or in the magnet itself which impede the rate in increase of the magnetic field. The magnetizer must be able to supply a high rate of energy transfer for a period of time long enough to overcome these retarding effects, but the pulse must not be too long because the current is very high, on the order of thousands of amps. If this high current were to be maintained for more than a very brief time, the fixture would overheat or perhaps even vaporize. It is critical, therefore, in such devices to precisely control both the intensity and the duration of the current pulse that creates the magnetizing field.
Magnetizers of the sort discussed above must magnetize various materials of differing sizes. This being the case it is necessary to precisely define the parameters of the current pulse that will create the necessary high intensity magnetic field without overstressing the magnetizing fixture.
This current pulse is usually created by electrical charges stored in banks of capacitors where the peak voltage is often set by adjusting the output of a transformer that is used in the rectifier to convert line AC voltage to DC. This DC voltage is used to charge the capacitors. The problem with this circuit is that its charging rate is high at first but drops off as the capacitor banks charge up, resulting in an increased period of time required to charge. This is undesirable because it may slow production.
When the capacitor banks become charged they are discharged through a fast-acting switch. In the past such devices often used a mercury-filled vacuum tube. This type of switch has a high and a variable voltage drop depending on its temperature and age and requires high voltage to fire. The switch may also present a health hazard in case of breakage due to the mercury and its vapor. Such switches are, therefore, highly unsatisfactory for this type of application.
Due to the widely varying types of motors and magnetic materials available today there is a need for magnetizers which can deal with the varied requirements for magnetic materials. For example, a particular magnetizer may have either too small or too large a capacitance. Regardless of the voltage the current pulse needed to create the magnetic field may thus be incorrect for that particular application. There is, therefore, a need for a magnetizer which has both a variable capacitance as well as a variable voltage in order to precisely define the intensity and duration of the high current magnetizing pulse.
It is also desirable to have a magnetizer which includes means of measuring the peak output current, If the magnetizing fixture begins to overheat due to a high cycle rate of usage, for example, or to short circuit within the fixture itself, the peak current will change. A change in peak current may indicate poor magnetization or progressive failure which may become catastrophic if left uncorrected. The current pulses which create the magnetizing fields, however, are of very short duration and very high amplitude, and are, therefore, difficult to monitor.